If you want GNOME to start automatically, try installing gdm. GDM is the default X display manager (which provides the login screen, and launches GNOME on login), and it should automatically start on boot.

In order to run gnome you must also install all the xorg/xserver stack as well as a lot of gnome pieces to get a functional desktop.

Normally you would just install ubuntu-desktop, which would install everything for you. But something in your question suggests what you want to do is install each package and see how it fits together to make a small install base. Run this:

apt-cache show ubuntu-desktop | grep Recommends:

And look through the results to find packages which you may need to install to get a gnome desktop working. Once you've got gdm and xorg installed it should just be a matter of:

sudo /etc/init.d/gdm start

And you can also make gdm start by default by adding it to the rc.d if it's not loading by default when you install it.